Hair styling using dielectric heating

ABSTRACT

In a hair styling device (100) comprising electrodes (102) for applying a radio-frequent signal to hair, a frequency of the radio-frequent signal is between 50 MHz and 90 MHz. Preferably, a voltage of the radio-frequent signal does not exceed 30 V, while a voltage not exceeding 10 V would work well in a configuration that applies another heat source (103) for heating hair up till a first temperature that is no more than 150° C. (and thus lower than a critical temperature at which hair cuticle damage will occur), and the radio-frequent electrodes (102) for in combination with heat from the other heat source (103)—selectively heating a hair cortex to a second temperature exceeding the first temperature and sufficiently high for hair styling.

FIELD OF THE INVENTION

The invention relates to a hair styling device using dielectric heating.

BACKGROUND OF THE INVENTION

US 2017/0360174 discloses an apparatus that manipulates the shape of hair using dielectric heating. Typically, the apparatus includes opposing first and second electrodes respectively provided on first and second arms that are movable towards and away from one another. Drive circuitry supplies electrical energy to the first and second electrodes, to cause an alternating electric field to be produced in the vicinity of the electrodes in use, and thereby cause dielectric heating of hair placed between the electrodes in use. Sensing circuitry senses a change in coupling of energy from the alternating electric field to the hair during heating of the hair. Control circuitry controls the drive circuitry to vary the electrical energy supplied to the first and second electrodes in dependence upon the sensed change in coupling.

The range of frequencies is preferably from around 1 MHz to around 100 MHz. More preferably the range of frequencies is from around 10 MHz to around 100 MHz. Even more preferably the range of frequencies is from around 20 MHz to around 40 MHz, these frequencies being well suited for consumer products since they have limited wave propagation (unlike microwaves) and hence do not present a risk to health or undesirable EMC (electromagnetic compatibility) effects. A voltage of around 50 V AC is used.

SUMMARY OF THE INVENTION

It is, inter alia, an object of the invention to provide an improved hair styling device using dielectric heating. The invention is defined by the independent claim. Advantageous embodiments are defined in the dependent claims.

Embodiments of the invention are based on the recognition that in the frequency range of below 40 MHz, as preferred by the prior art US 2017/0360174, the loss tangent and heat generation significantly depend on the moisture content. So, with frequencies below 40 MHz, as preferred by the prior art, the power levels need to be adapted depending on the water content of the hair. Moreover, since the water content changes significantly during the styling process, heat control requires real-time accurate monitoring of water content to provide optimal heating effects, to prevent undesirable over-heating effects and poor efficacy due to under-treatment from occurring. In that sense, it is desirable to have a method that is less dependent on the water content and which requires less critical sensing and control of treatment parameters. In accordance with an aspect of the invention, this can be achieved by using frequencies in the range from 50 MHz to 90 MHz which will make the treatment less dependent on water content and at the same provides about the same efficacy as around 1 MHz for dry hairs. Preferably, a voltage of the radio-frequent signal does not exceed 30 V, while a voltage not exceeding 10 V would work well in a configuration that applies another heat source for heating hair up till a first temperature that is no more than 150° C. (and thus lower than a critical temperature at which hair cuticle damage will occur), and radio-frequent electrodes for in combination with heat from the other heat source—selectively heating a hair cortex to a second temperature exceeding the first temperature and sufficiently high for hair styling.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of loss tangent vs. frequency; and

FIG. 2 shows an embodiment of a hair styling device in accordance with the present invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a graph of loss tangent vs. frequency, for wet hair W and dry hair D, respectively. The graph shows a relatively large difference in loss tangent between wet hair W and dry hair W for frequencies below 40 MHz, while the difference is much smaller for higher frequencies above 50 MHz, so that at such higher frequencies, it is not necessary to accurately and constantly measure the moisture content of the hair while styling it. While the graph shows that the same advantage also occurs at frequencies higher than 90 MHz, an efficacy of the RF heating is less at frequencies exceeding 90 MHz.

FIG. 2 shows an embodiment of a hair styling device 100 in accordance with the present invention, similar to our previous application WO 2017/080957 (attorneys' docket 2015PF01581), with the difference that in the embodiment of FIG. 2, radio-frequency (RF) heating is used instead of light. The hair styling device 100 comprises two jaws 101 which are hinged to pinch strands of hairs. Each jaw 101 comprises an array of RF electrodes or large area electrodes 102, which will selectively heat the hair cortex to a temperature of about 170° C. required for styling. This temperature can be controlled to prevent other non-desirable heating effects such as melting and vaporization expected at 250° C. and 1200° C. respectively.

Each jaw 101 comprises optionally another heat source 103 which may directly or indirectly pre-heat the hairs to a first temperature of no more than 150°, preferably no more than 140° C., e.g. 120° C., to enhance the dielectric properties of the hair. The combined heating effect from the other heat source 103 and the RF electrodes 102 will lead to the temperature of about 170° C. required for styling.

Additional hair conductivity sensing controls 104 can be used to measure hair dielectric properties and optimize the treatment by adjusting the power. Additional heat control systems 105 can be used to regulate the temperature settings and treatment time. To straighten/style the hairs, the strands of hairs are inserted between the plates and are moved until the hair is styled.

Contact resistive RF heating is preferred over non-contact capacitive RF heating for a consumer proposition because of the high voltage and power or longer cutting time required for the latter one. A voltage below 10 V is suitable for use with high frequencies in the range between 50 MHz and 90 MHz in the embodiment of FIG. 2.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. For example, while the embodiment shown in FIG. 2 is a hair straightener, the invention can alternatively be used for hair curling. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and/or by means of a suitably programmed processor. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. Measures recited in mutually different dependent claims may advantageously be used in combination. 

1. A hair styling device comprising electrodes for applying a radio-frequent signal to hair, characterized in that a frequency of the radio-frequent signal is between 50 MHz and 90 MHz.
 2. The hair styling device as claimed in claim 1, wherein a voltage of the radio-frequent signal does not exceed 30 V.
 3. The hair styling device as claimed in claim 2, wherein the hair styling device further comprises heat source different from the electrodes for heating hair up till a first temperature that is no more than 150° C., and wherein a voltage of the radio-frequent signal does not exceed 10 V. 